Abstract
Loading schedule during spark-plasma-sintering (SPS) processing strongly affected the densification and the optical transmission of MgAl2O4 spinel ceramics. By loading at room temperature prior to heating, high density transparent spinel ceramics were attained only at slow heating rates of ≤10 ˚C/min. By loading at high temperatures, however, spinel ceramics attained reasonable high density and resultant optical transmission even at a high heating rate of 100 ˚C/min though the transmission is lower than those attained by the slow heating SPS processing. The loading dependent densification can be ascribed to closed pore formation mechanisms. This suggests that during the SPS processing, the load controlling is one of the important key factors as well as the heating rate and sintering temperature. In order to fully utilize the high heating rate, that is the primary advantage of the SPS processing, the loading schedule should be optimized on the bases of the densification mechanisms. Depending on the loading schedule, discoloration of the SPSed spinel ceramics also changed and was ascribed mainly to carbon contamination. The optical transmission of the SPSed spinel ceramics was restrained by the carbon contamination, and hence, lowered than the highest value reported by the HIP technique.
